Basic Heavy Rain and Flash Flood Concepts

Prolonged heavy rain from low pressure storm systems and especially thunderstorms can pose a potential flash flood threat. Flash flooding is a serious threat to life and property requiring immediate action if flooding is occurring or imminent. Never drive across flooded roads. Move to higher ground if necessary. Be particularly careful at night when it is difficult to see or assess flood waters. NWS Louisville staff members monitor heavy rain and flash flood potential during significant storm systems. The WSR-88D Doppler radar can estimate rainfall. When Louisville forecasters believe that flash flooding is imminent or if a flash flood report is received, then they issue a flash flood warning for the area concerned. Below are some basic heavy rain and flash flood concepts.

A simple rainfall concept:

The heaviest precipitation occurs where the rainfall rate is the highest for the longest period of time.

Factors that affect heavy rainfall production:

Precipitation efficiency: How efficient is the thunderstorm in converting water vapor condensates into rainfall that reaches the ground.

Rainfall rate: How intense is the rainfall at any one spot.

Factors that contribute to efficient rainfall production and high rainfall rates:

b. Slow moving/Backward/Regenerative propagation: individual convective cells move forward but continued cell redevelopment upstream causes a thunderstorm complex to exhibit little or no overall (net) movement (i.e., some cell movement but little or no system movement); flash flooding is very possible in these situations.